The invention relates to an osteosynthesis device, in particular a pedicle screw.
Osteosynthesis devices of this type are well known from the prior art. Osteosynthesis devices relate in particular to the field of spinal surgery, in order to align adjacent vertebral bodies and fix said bodies in a desired position relative to one another. In the process, a bone anchor is inserted into each adjacent vertebral body, specifically is typically screwed into said body in the form of a bone screw, and these adjacent bone anchors are then connected by a correction element, typically by a correction rod, which is clamped in the fork head of a relevant osteosynthesis device in a particular position desired by a surgeon. In this way, adjacent vertebral bodies are aligned and fixed relative to one another in a desired position.
An osteosynthesis device of this type is known from EP 2 301 458 A1, for example. In this bone screw known from the prior art, the bone anchor or bone screw is inserted through a receiving opening from the proximal end area of the fork head, such that the head is mounted so as to pivot in a tapered area of the receiving opening. In order to fix the bone screw in a particular angular position determined by the surgeon, a thrust piece is provided that is inserted into the fork head from the proximal end. When fitting the bone screw, this thrust piece is moved towards the distal end area of the fork head by the correction element or correction rod by screwing a set screw into a thread of the fork head, such that said thrust piece fixes the head of the bone anchor or bone screw in the appropriate angular position relative to the fork head. In order to loosen this fixation in an angular position, it is sufficient to loosen the set screw such that the axial pressure that the correction element or correction rod exerts on the thrust piece can be taken away from the thrust piece such that the thrust piece no longer clamps the head of the bone anchor.
However, a drawback of osteosynthesis devices of this type is that the shank diameters of the bone anchors or bone screws that comprise an external thread for screwing into a bone or vertebral body are limited. It is thus not possible to insert a bone anchor or bone screw that has a shank diameter that is greater than the diameter of the head into the receiving opening from the proximal end of the fork head. Therefore, for applications that require a large shank diameter, osteosynthesis devices of this type cannot be used.
The prior art also discloses osteosynthesis devices in which a head of a bone anchor or bone screw is inserted into the fork head of the osteosynthesis device from a distal end. An osteosynthesis device of this type is known from EP 2 570 090 B1 and US 2009/0036934 A1. In this osteosynthesis device, the fork head is formed in two pieces, said head comprising a first proximal part having the arms for receiving the correction rod and comprising a distal clamping part for receiving the head of the bone screw, the outer surface of which clamping part is conical. The distal part is rotatably connected to the proximal part by means of two pins. The distal clamping part comprises resilient spring elements that delimit a receiving opening for the head of the bone screw. The head of the bone screw is inserted into the distal clamping part from the distal end, a clamping ring arranged radially outside the distal clamping part being provided for securing the head. When a surgeon is fitting this osteosynthesis device, a correction rod is again inserted between the arms of the proximal part and is moved by a set screw towards the distal end. This correction rod in turn contacts the clamping ring such that it presses the conical outer surface of the clamping part downwards such that the head of the bone screw is fixed in a particular angular position.
However, a drawback of an osteosynthesis device of this type is that it is not possible, or is only possible with difficulty, for a surgeon to correct the angular adjustment once the set screw has been tightened for the first time, since the clamping ring is already clamped on the conical outer surface of the clamping part. Loosening the set screw thus does not directly cause the angular position of the bone screw relative to the fork head to be loosened. This proves to be problematic in particular in bone screws that have already been screwed into bones or vertebral bodies. Furthermore, the multiple-part design of the fork head is complicated to fit and expensive to manufacture.